Unlike the earliest versions, the modern periodic table is in order by this number.
What is the atomic number?
100
Each "row" of the periodic table is more appropriately called this.
What is a period?
100
The collective name for the elements that border the "staircase," except for aluminum.
What are metalloids?
200
Another name for a column on the periodic table, elements in the same one of these have similar physical and chemical properties.
What is a group (or family)?
200
The alkaline earth metal in the third period.
What is Mg (magnesium)?
200
Of barium, cobalt, and fluorine, the one that is the most metallic.
What is barium?
300
Of the three broad classes of elements, the ones in this class are only "pretty good" conductors of electricity.
What are the metalloids (or semiconductors)?
300
One of the two halogens that is a solid at room temperature.
What are iodine or astatine?
300
If you subtract the atomic number of the alkaline earth metal in the second period from that of the noble gas in the third period and use it as an atomic number, you have identified this element.
What is silicon?
400
The only nonmetal to the left of the "staircase."
What is hydrogen?
400
An example of a rare earth metal.
What is any one of the inner transition metals?
400
An example of an element (by symbol or by name) in an "A" group that's not a main group element.
What is any one of the noble gases?
500
The father of the periodic table.
Who is Mendeleev?
500
An example of a noble gas that can form at least a few compounds.
What is any one of Kr, Xe, or Rn?
500
The two elements in the sixth period whose names differ by only a single letter.